1. Field of the Invention
The present invention relates to a surface acoustic wave filter that can selectively extract a desired frequency band by exciting a surface acoustic wave on a substrate made of a piezoelectric material, and more particularly to a resonator ladder type surface acoustic wave filter suited for use as a high-frequency filter in mobile communication apparatus of which sufficient attenuation of harmonic frequencies is required.
2. Related Art of the Invention
For many years, surface acoustic wave filters have been used widely as high-frequency filters in mobile communication applications. Among others, resonator ladder type surface acoustic wave filters have been receiving attention because of their low insertion loss, excellent 50 .OMEGA. matching, etc.
A prior art resonator ladder type surface acoustic wave filter will be described below. The basic principle of the resonator ladder type surface acoustic wave filter is the same as that of conventional ceramic ladder type filters, the only difference being that the resonators in the latter filters are replaced by surface acoustic wave resonators. In this type of filter, by making the resonant frequency of a series arm surface acoustic wave resonator substantially coincide with the antiresonant frequency of a parallel arm surface acoustic wave resonator, a filter characteristic is obtained that has a pass band between these frequencies and stop bands above the antiresonant frequency of the parallel arm surface acoustic wave resonator and below the resonant frequency of the series arm surface acoustic wave resonator.
The basic configuration of the prior art resonator ladder type surface acoustic wave filter is shown in the circuit diagram of FIG. 6, in which series arm surface acoustic wave resonators and parallel arm surface acoustic wave resonators are alternately connected on a piezoelectric substrate; this piezoelectric substrate is housed in a package and electrically connected by bonding wires, as shown in FIGS. 7A and 7B. Here, reference numeral 1 designates the piezoelectric substrate, 2 an input electrical terminal, 3 an output electrical terminal, 5 the series arm surface acoustic wave resonators, and 6 the parallel arm surface acoustic wave resonators. Further, 20 is a casing, 21 is a cover, 22 is a grounding lead, and 23 is a plate.
This resonator ladder type surface acoustic wave filter has a frequency characteristic such as shown in FIG. 8.
As can be seen from the frequency characteristic of FIG. 8, the prior art resonator ladder type surface acoustic wave filter achieves stable and good out-of-band rejection near the pass band, but in harmonic frequency regions such as the regions of the second and third harmonics, the out-of-band rejection rapidly decreases, thus degrading the filter characteristic. The reason for this is believed to be that the inductance associated with the strip lines connected to the parallel arm surface acoustic wave resonators 6 on the piezoelectric substrate 1, as well as the inductance associated with the strip lines and bonding wires within the package, causes a phenomenon similar to resonance in the harmonic frequency regions by interaction with the parallel arm surface acoustic wave resonators 6. To reduce this phenomenon, either the above inductances or the capacitances of the parallel arm surface acoustic wave resonators 6 must be reduced.
However, since the capacitances of the parallel arm surface acoustic wave resonators 6 are closely related to the out-of-band rejection characteristics near the pass band that are determined by electrical matching with filter external circuits and the capacitance ratio between the series arm surface acoustic wave resonators 5 and the parallel arm surface acoustic wave resonators 6, it is not possible to reduce the capacitances of the parallel arm surface acoustic wave resonators 6 arbitrarily.